Split() clean up locking

Only unlock and relock when idle_loop() is actually called

No functional change

src/thread.cpp

@@ -303,28 +303,28 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
       slave->notify_one(); // Could be sleeping
   }
 
-  sp.mutex.unlock();
-  Threads.mutex.unlock();
-
   // Everything is set up. The master thread enters the idle loop, from which
   // it will instantly launch a search, because its 'searching' flag is set.
   // The thread will return from the idle loop when all slaves have finished
   // their work at this split point.
   if (slavesCnt > 1 || Fake)
   {
+      sp.mutex.unlock();
+      Threads.mutex.unlock();
+
       Thread::idle_loop(); // Force a call to base class idle_loop()
 
       // In helpful master concept a master can help only a sub-tree of its split
       // point, and because here is all finished is not possible master is booked.
       assert(!searching);
       assert(!activePosition);
-  }
 
-  // We have returned from the idle loop, which means that all threads are
-  // finished. Note that setting 'searching' and decreasing splitPointsSize is
-  // done under lock protection to avoid a race with Thread::is_available_to().
-  Threads.mutex.lock();
-  sp.mutex.lock();
+      // We have returned from the idle loop, which means that all threads are
+      // finished. Note that setting 'searching' and decreasing splitPointsSize is
+      // done under lock protection to avoid a race with Thread::is_available_to().
+      Threads.mutex.lock();
+      sp.mutex.lock();
+  }
 
   searching = true;
   splitPointsSize--;